This invention relates to dry castable concrete compositions, a method of preparing these compositions, and dry casting methods using these compositions. More particularly, this invention relates to dry castable concrete compositions containing condensed silica fume and casting these compositions to produce sewage pipe having improved strength and resistance to acids and other aggressive chemicals.
It is well known to produce sewage pipe and concrete pipe for general purposes by dry casting methods. These methods are so called because they use a dry concrete mix, i.e., one containing a relatively low amount of water and having a low slump. Ideally, the mix contains only that amount of water which is necessary for hydration of the cement and desired workability. The water to cement ratio is usually less than 0.30 and the slump is less than 1/2 inch, preferably zero or less.
There are four conventional dry casting methods used in the industry. These are commonly referred to as the packerhead, dry cast, tamp, and centrifugal or spinning methods. The following descriptions of these methods are extracted in part from the "Concrete Pipe Handbook", published by the American Concrete Pipe Association, Vienna, Va., 1981 ed., which is incorporated by reference herein:
1. Packerhead process--This process uses a stationary exterior form which is usually cylindrical. The dry concrete mix is fed into the form and, as it is introduced, a revolving disc or "packerhead", extended from a revolving shaft, is drawn upward through the form. The disc diameter determines the inside diameter of the pipe. The head has rollers or deflectors mounted on the top which compact the mix against the form. When compaction is complete, the form and pipe are moved to a curing area where the exterior form is removed. PA1 2. Centrifugal or spinning process--"The centrifugal or spinning process uses an outer form that is rotated in a horizontal position during the pipe making process. Vibration and compaction can be used in combination with centrifugation to consolidate the concrete mix. While the mix is wetter than some pipe mixes, water is extracted from the concrete by the centrifugal forces which develop as the pipe is spinning. As the form is rotated, concrete is fed into the form by a conveyor system that is capable of distributing concrete throughout the form length. This finished pipe, still in the form, is moved to the curing area and cured in the form." PA1 3. Dry cast Process--"The dry cast process has several variations but all use low frequency-high amplitude vibration to distribute and densely compact the dry mix in the form. The form is removed immediately as the newly formed pipe can support itself. To get the desired vibration at all points, several different techniques are utilized. PA1 4. Tamp process--"The tamp process uses direct mechanical compaction to consolidate the concrete mix. Inner and outer forms are placed on a rotating table, and the concrete mix is fed into the forms. As the form is rotated and filled the tamper rises automatically. There are usually multiple tampers so that the mix on each side of any reinforcement can be compacted. The pipe is removed from the machine with either the inner or outer form and moved to the curing area where the form is removed."
In one method, vibrators are usually attached directly to the exterior form. The mix is fed into the form and the vibrators are operated at various stages during this process. At the completion of the process, the pipe and form is lifted off the machine and moved to the curing area where the form is removed.
Another variation of the dry cast method has a central core that moves up and down and provides vibration and compaction. In this method, when the process is completed, the core retracts and the pipe is stripped and moved to the curing area."
Although concrete is a preferred material for preparing sewage pipe, it has the disadvantage of being subject to attack by sulfuric acid and other chemicals in the sewage. The sulfuric acid can result from the oxidation of hydrogen sulfide which in turn results from the action of anaerobic sulfur-reducing bacteria. The sulfuric acid reacts with the alkaline cement, particularly the calcium hydroxide component, resulting in deterioration and cracking of the concrete.
It is an object of this invention to provide a method of producing concrete sewage pipe having improved chemical resistance, particularly to sulfuric acid attack.
It is another object of this invention to provide a method for producing chemically resistant concrete pipe by dry casting methods.
It is another object of this invention to provide a dry concrete mix which is suitable for use in preparing pipe by dry casting methods and after setting has improved strength and chemical resistance.
It is another object of this invention to provide a method for preparing the dry concrete mixes of the invention.